Wide band-gap III-V nitride materials are recognized to be among the most attractive compound semiconductors for use in a variety of devices. They are suitable for optoelectronic and microelectronic devices that operate in a wide spectral range, from visible to ultraviolet, and in the high temperature/high power applications area. The main advantage of nitride semiconductors in comparison with other wide-band-gap semiconductors is their low propensity to degrade at high temperature and high power when used for optical and microelectronic devices.
Current commercial nitride semiconductor products are typically based on c-axis polar III-V nitride materials. In c-axis-oriented hexagonal III-V nitride material, the spontaneous and strain-induced piezoelectric polarizations produce strong electric fields which result in band bending of the quantum wells and spatial separation of electrons and holes. Moreover, these properties remain even at the high carrier densities required for laser operation, as has been previously described in the art. (See, e.g., I. H. Brown et al, IEEE J. Quant. Elec. 42 1202, 2006). Consequently, when using these conventional materials the radiative recombination time increases at the expense of quantum efficiency and a red-shift of the emission occurs. Accordingly, non-polar III-Nitride materials, where these polarization fields and the resulting band bending are absent, would open up the possibility of fabricating high efficiency and high power UV-visible light emitters.
Epitaxial lateral overgrowth techniques (“ELOG”) and its modifications, such as facet initiated epitaxial lateral overgrowth (“FIELO”) and Pendeo (from the Latin to hang or be suspended) are the most widely used approaches employed for suppressing bending and the threading dislocations associated with these materials. Indeed, laterally overgrowing oxide (or metal) strips deposited on initially-grown GaN films has been shown to achieve about two orders of magnitude reduction in dislocation density, reducing it to the 107 cm−2 level. However, the low defect-density material only occurs in the wing region, located in the coalescence front, and represents only approximately one fifth of the whole wafer surface area. Large coalescence front tilting and tensile stress are both present in the overgrowth region. The same ELOG and Pendeo methods could also be used to reduce the defect density in non-polar GaN (on a- or m-plane), although the wing tilt resulting from the different growth rates of the Ga-polar and N-polar wing in these materials introduces further complexity at the coalescence boundaries.
Low defect-density free-standing c-axis polar GaN is currently one of the materials of choice to achieve the desired specification for optoelectronic and microelectronic devices. However, the use of non-polar III nitride materials for semiconductor devices such as high current density drive light-emitting diodes (LEDs) and laser diodes (LDs) may disrupt the current trend for polar materials. Bulk (melt or sublimation) and hydride vapour phase epitaxy (HVPE) are the two main techniques for growing free-standing and low defect-density c-axis polar and non-polar III nitride semiconductor materials. Bulk GaN growth techniques operating at a very high pressure of ˜15 kbar have been successful in growing low dislocation density (<107 cm-2) material. Unfortunately, this technology suffers from a low growth rate and is limited to small diameter substrates, making these materials very expensive and uneconomic for commercial manufacturing.
HVPE is a reversible equilibrium-based hot-wall process with several advantages, including, high growth rate of up to about 400 μm/hr—more than 100 times faster than that of the metalorganic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) methods, low running costs, and lower defect densities in thick GaN as the result of the mutual annihilation of mixed dislocations. However, the HVPE technique still has the same inherent problems as other growth techniques due to its growth on foreign substrates. Therefore, the growth of thick GaN using HVPE in general has to overcome two critical issues, firstly, to reduce the bending and cracking of initial GaN thick films (30-100 μm) on foreign substrates, and secondly, to minimize the defect density of GaN.
To reduce defect density (mainly threading dislocations) and strain, and to improve the surface morphology of the thick GaN films grown by HVPE, various techniques have been employed. For example, ELOG growth under lower reactor pressure and growth with TiN intermediate layers, porous SixNy layers, AlN/GaN superlattices, III nitride semiconductor nanopillars, or deep inverse pyramid etch pits on weakened Si, GaAs and other III-V single crystal wafers have all been used to try and address surface morphology. Various defects reduction methods using defects filter layers, porous interlayers and III nitride semiconductor nanopillars are described in U.S. Pat. Nos. 6,835,246; 6,596,377 and 6,844,569, as well as US Patent Publication Nos. 2004/0157358, 2006/0270201, 2003/0006211, 2002/0111044, 2004/0123796, 2004/0251519 and 2001/0055881. Additional exemplary disclosures may be found in foreign patent publication Nos. JP 2005136106, WO 02/44444, EP 1246233, and EP 1422748, as well as the following non-patent literature R. F. Davis et al, ‘Review of Pendeo-Epitaxial Growth and Characterization of Thin Films of GaN and AlGaN Alloys on 6H—SiC(0001) and Si(111) Substrates’, MRS Internet J. Nitride Semicond. Res. 6, 14, 1(2001); K. Kusakabe, A. Kikuchi, and K. Kishino, ‘Overgrowth of GaN layer on GaN nano-columns by RF-molecular beam epitaxy’, J. Crystl. Growth, 237-239, 988 (2002); C. C. Mitchell et at., ‘Mass transport in the epitaxial lateral overgrowth of gallium nitride’, J. Cryst. Growth., 222, 144 (2001); K. Hiramatsu., ‘Epitaxial lateral overgrowth techniques used in III-V nitride epitaxy’, J. Phys: Condens, Matter., 13, 6961 (2001) and R. P. Strittmatter, ‘Development of micro-electromechanical systems in GaN’, PhD Thesis, California Institute of Technology, P. 92 (2003). The disclosure of each of these references is incorporated herein by reference.
However, growth processes using these techniques are tedious, time consuming and expensive. Moreover, the GaN produced using such techniques still has the major disadvantage of being subject to bending and undesired residual strain. Accordingly, improved methods of producing non-polar III nitride materials having both low defect density and low stress are needed.